The present invention relates to partial oxidation of organic compounds in the gas phase to produce valuable compounds on supported and immobilized photocatalysts which have been deposited using a flame aerosol coating method.
The cost of handling, treating and disposing of wastes generated annually in the United States has reached as much as 2.2% of the gross domestic product, and continues to rise. The chemical manufacturing industry generates more than 1.5 billion tons of hazardous waste and 9 billion tons of non-hazardous waste annually. Roughly half of the releases and transfers of chemicals reported through the Toxic Release Inventory and 80-90% of hazardous waste generation reported through the Resource Conservation and Recovery Act (RCRA) are due to chemical manufacturing. Organic chemicals comprise the largest amount of toxic release. Many of these releases can be minimized by using better management of materials and energy, more efficient process control, optimizing process conditions, and recycling and reusing waste and byproducts. However, cleaner products methods can be achieved by adopting xe2x80x9cgreen syntheticxe2x80x9d methods.
Oxidation reactions are used in industry for producing aliphatic and aromatic aldehydes, alcohols, ketones and carboxylic acids. Generally, oxidation involves splitting of Cxe2x80x94C or Cxe2x80x94H bonds with concomitant formation of Cxe2x80x94O bonds. For example, the partial oxidation of hydrocarbons by molecular oxygen to form oxygenates, which are further used as building blocks in manufacturing plastic and synthetic fibers, is an important process in the chemical industry. Oxidation reactions are usually catalyzed and are carried out in liquid or gas phase. The current processes are energy intensive, have low conversion coefficients, and generate environmentally hazardous waste and byproducts.
Current processes for producing these highly desired oxygenates require stringent operating conditions such as high temperatures and pressure, strong acids, free radicals (halogenated starting materials) and corrosive oxidants. Although these processes are currently being used, they have low energy efficiencies and generate environmentally hazardous waste and byproducts. One major reason selectivities are low is that the desired products, such as carbonyls and alcohols, are more easily oxidizable by oxygen than is the parent hydrocarbon. Over-oxidation must be minimized by maintaining conversions low, which is a serious disadvantage from the viewpoint of chemical processing and economics. Therefore, a major challenge is to find reaction pathways that produce the primary product with high selectivity and at high conversion rates for the hydrocarbons.
Most oxidations are highly exothermic and may generate high localized temperatures and hot spots on the catalyst surfaces, decreasing the service lifetime of the catalyst and resulting in over oxidation of desired products. Over oxidation can be minimized only by keeping conversions low. Therefore, a major challenge in this field is to find a reaction pathway that affords the primary product with high selectivity and a high conversion rate of the hydrocarbons. There is a strong research effort underway to meet this challenge. Unfortunately, several commonly used catalysts for oxidation reactions are toxic heavy metals such as chromium and vanadium, or strong acids such as sulfuric acid or nitric acid. Pollution is inevitable in loading, recovering, and regeneration of these catalysts. A cleaner alternative is needed.
Photocatalytic or photoactivated reactions are applicable to a wide range of valuable industrial processes, including organic synthesis, photodestruction of toxic compounds, and purification of drinking water. The anatase form of TiO2 has been the most extensively used in photocatalytic reactions because of its high activity and chemical stability. For example, the anatase phase of titania can be used as a photocatalyst for several problems of environmental interest, as a catalyst for sulfur removal, for toxic metals capture, and as an additive in cosmetics because of its effective sunscreen properties.
The electronic structure of titania is characterized by a filled valance band and an empty conduction band. When a photon with energy exceeding the band gap energy is incident, an electron is readily excited to the conduction band, leaving a hole in the valence band. If surrounding and surface conditions are correct, the excited electron and hole pair can participate in reduction-oxidation reactions. The quantum efficiency of the semiconductor photocatalyst depends on the recombination lifetime of holes and electrons, and the rate of interfacial charge transfer. Therefore, crystal structure, grain size, and attendant surface morphologies can affect quantum efficiencies.
Recently, titania has been used in an atmospheric pressure process for coating steel substrates to provide for stainless and corrosion resistance characteristics. Several researchers have been studying different applications for titania films, relying on its self-cleaning and superhydrophilic properties. The feasability of this technology on a commercial scale has also been demonstrated by the implementation of numerous small-scale applications for treating contaminated air and water streams.
Photocatalytic oxidation of many organic molecules, including saturated hydrocarbons, by optically excited semiconductor oxides is thermodynamically feasible in the presence of oxygen at room temperature. UV light-assisted oxidation has been shown to be promising for oxidizing cyclohexane and epoxidizing small olefins. Selectivities different from those obtained by other oxidation routes have been reported, illustrating the potential of the method for syntheses, provided that that the expected product can be obtained with an acceptable yield.
The rate of photooxidation is affected by the solvent type, colloidal preparation, the oxygen concentration, catalyst surface area, and light intensity. Aromatic aldehydes have been synthesized by oxidating the methyl groups of toluene and derivatives by routes which can also yield the corresponding alcohols and carboxylic acids. Various factors controlling the yield and specificity of the reaction products have been investigated, as the photophysics of the various excited states reactions occurring at the titanium dioxide/non-aqueous solution interfaces. Such additional thermodynamic control or redox chemistry for systems containing the semiconductor catalysts has been previously ascribed to differences in that the adsorption of the various oxidizable species to be adsorbed on semiconductor surfaces.
Most studies related to synthesizing compounds using titania catalysts have been performed in the liquid phase. Sahle-Demiessie et al., in Ind. Eng. Chem. Res. 1999, 38, 3276-3284, describe the activation and oxygenation of hydrocarbons in the gas phase by irradiating supported titania films with ultraviolet irradiation. The oxidation in the gas phase eliminates an additional separation step, as the catalyst is supported. Product adsorption can also be minimized by using slightly elevated temperatures. Furthermore, operating conditions such as feed rates, humidity, mixing conditions, and residence times can be readily controlled. One concern, especially in gas-phase reactors for complete oxidation to carbon dioxide, has been the formation of byproducts that lead to catalyst deactivation. For wide-scale applicability and commercialization of this technology, it is essential to develop reactors with well-controlled thin films that are not readily deactivated.
Designing the photocatalytic reactor with films of controlled characteristics is thus an important consideration for practical application. Several different methodologies have been used to produce titania powders and films, including wet processes, sol-gel processes, and gas phase processes. Gas phase coating processes have the advantage that no toxic liquid waste byproducts are produced, and, moreover, these properties can be controlled so as to obtain the desired properties. Various dry coating methods such as ion plating, sputtering, plasma chemical vapor deposition, chemical vapor deposition, and physical vapor deposition have been developed for the electronics industry. However, most of these processes are low pressure processes and are either not feasible or are too expensive for coating larger areas. Also, the film composition and other properties cannot readily be varied using these coating methods.
Yang et al. have developed atmospheric pressure coating methods for applying ceramic coating onto steel and quartz substrates, and have obtained superior adhesion and corrosion resistance properties. Using flame aerosol coating, films of 5 nm to 1 micron were deposited onto stainless steel and silica substrates.
Tabatabaie-Raissi et al., in U.S. Pat. Nos. 5,604,339 and 5,744,407, describe methods for destroying toxic volatile air-borne toxins using a chemical membrane based on titania which acts as a photocatalyst for inhibiting emissions of harmful volatile organic compounds. A suspension of titania in water is coated onto a substrate to produce such a membrane.
Edwards et al., in U.S. Pat. No. 4,152,230, disclose a photochemically initiated oxidation process for organic chemicals by conducting the reaction in the presence of a compound of a multiple valent metal, especially copper, to trap the photochemically produced radicals. In this case the metal must possess a reduction potential which is not so high that the compound would oxidize the organic substrate on its own without the initial hydrogen abstraction process effected by the photochemically activated species. Among the metals that can be used are copper, iron, cobalt, manganese, chromium, and vanadium.
Juillet et al., in U.S. Pat. No. 3,781,194, disclose a process for photocatalytic oxidation of hydrocarbons into aldehydes and ketones. Among the catalysts that can be used for this process are titania, zirconia, and magnesia.
It is an object of the present invention to overcome the aforesaid deficiencies in that the prior art.
It is another object of the present invention to synthesize high-value organic compounds using photocatalytic oxidation with semiconductor material.
It is a further object of the present invention to provide a process for partially oxidizing organic chemicals to alcohols, ketones, and aldehydes, using flame deposited nanostructured photocatalysts.
It is another object of the present invention to activate and oxidize hydrocarbons using light energy and a specially prepared catalyst.
It is yet another object of the present invention to produce industrially useful products at high selectivity while producing minimal byproducts and pollutants.
According to the present invention, high value organic compounds are synthesized using photocatalytic oxidation with a flame-deposited semiconductor material such as titanium dioxide. The process of the present invention can be applied to a variety of hydrocarbons which can be oxygenated in both liquid and gas phase using ultraviolet light and a semiconductor under mild conditions.
According to the present invention, gas phase photocatalytic oxidation reactions of hydrocarbons are effected by flowing a known mixture of heated humid air along with the organic vapor through a reactor, preferably an annular reactor. The inner wall of the reactor is coated with a semiconductor film which had been deposited using a flame aerosol method. The efficacy with which the photoreactor operates is influenced by the oxygen concentration, the light illumination, the properties of the photocatalytic coating, and the conditions within the fluid phase affecting contact to the substrate to the titania surface.
In particular, air and substrate are reacted at ambient conditions in a gas phase photocatalytic reactor that uses ultraviolet light, preferably of solar origin, and a specifically prepared semiconductor catalyst, such as titanium dioxide.
The present invention provides clean production technology by selectively producing partial oxygenates and producing less by-products and pollutants than conventional oxidation reactions. The present invention incorporates the idea of xe2x80x9catom economyxe2x80x9d by direct oxygenation of hydrocarbons without using multiple stages or loss of atoms.